This invention relates to a balanced vane type oil pump and, more particularly, to an oil pump wherein a pump cartridge is used as two pumps and the supply of pressurized liquids from both pumps is selectively controlled to reduce consumption.
For example, an oil pump used as a source of pressurized oil of a power steering apparatus utilized to decrease the steering effort required of a driver of a motor car is usually driven by the engine of the car so that the output of the pump varies in proportion to the number of revolutions of the engine. Accordingly, such pump should have a capacity sufficient to insure satisfactory operation of the power steering apparatus or any similar liquid pressure apparatus even in a low speed operational range of the engine at which time the output quantity of the pump is small.
However, when the pump capacity is set in this manner, an excessive quantity of pressurized oil would be supplied to the liquid pressure apparatus during high speed engine operation, which is not only useless but which also increases the engine horsepower required for driving the pump, resulting in a larger consumption of fuel.
To solve this problem, it has been proposed to use a pair of pump cartridges as two pumps, each having a small capacity, and to combine a control unit with the pump cartridges. The control unit acts as a flow path switching mechanism for selectively supplying the outputs of both pumps to the liquid pressure apparatus. Thus, according to this arrangement, when the outputs of the respective pumps are small they are combined for use, whereas when the outputs increase, the output of only one pump is supplied to the power steering apparatus or similar pressure liquid apparatus and the output of the other pump is returned to the oil reservoir, thus circulating the oil through the other pump without utilizing the work generated by the pump resulting in wasted power consumed by the pump.
The arrangement described above is constructed such that it switches the flow paths based on the output of the respective pumps as well as on the speed of the engine, so that although it is possible to decrease the power consumption during the high speed operation of the vehicle, in the low speed range, the power consumption by both pumps is not avoidable.
Thus, this arrangement involves many problems which must be solved.
More particularly, in the power steering apparatus of the type described above, the quantity of the pressurized oil presents a problem under a high load state requiring larger output of the pump, that is, when the power steering is used at low speed and when the car is not running or running on a straight road. Further, the quantity of the pressurized oil supply is also small when the engine is running in the low speed range. Usually, in this situation the car is operating in a city, for example, in ten mode running patterns, in which it is necessary to reduce power consumption due to the low speed operation.
For this reason, it is advantageous to use a control unit having a flow path switching mechanism that senses the load of the power steering apparatus. Such a mechanism, however, presents the problem that even when the engine runs at a high speed so that the output of only one pump is sufficient, the flow path switching in response to load requires the use of both pumps, thus increasing power consumption.
It has also been proposed to use a flow path switching mechanism in which the running speed of the vehicle is electrically detected and the detected electric signal is utilized for effecting the flow path switching. However, since the vehicle speed is not always proportional to the number of revolutions of the engine, that is the pump output, this mechanism, responding solely to vehicle speed, cannot efficiently decrease the power consumption. A truck carrying excessive load may be running at a low speed with the engine operating in the high speed range, thus reducing pump output when it is required the most. Furthermore, such a flow path switching mechanism requires the use of electrical sensors and an electromagnetic valve operated thereby which is expensive.
The control unit described above to control the supply quantity of the pressurized liquid is required to selectively switch the flow paths from both oil pumps when desired, and to control the quantity of fluid to be supplied to the liquid pressure apparatus to a predetermined level. These two requirements are generally accomplished by using a pair of spool valves and a number of pressure liquid flow passages adapted to suitably combine these valves. Such construction, however, increases the cost of manufacturing and assembling the oil pump because the pair of spool valves and the flow paths are all incorporated in a single pump body together with a pair of pump cartridges.
There is also a constructional problem when a pair of pump cartridges consisting of rotors including vanes and cam rings are used as two pumps.
More particularly, for the purpose of utilizing a pair of pump cartridges as two pumps, the simplest construction is to use a pair of pump chambers formed at positions symmetric with respect to the rotor axis with the pump chambers independently connected to individual output passages. Examples of such constructions are disclosed in Japanese Laid Open Patent Specification Nos. 82868/1980 and 49594/1980 (U.S. Pat. No. 4,289,454). Although such constructions can simplify the layout of the pump passages and of the control unit, when the output of one pump chamber is connected to the reservoir side for unloading of same so that the pumping action is performed by only the other pump chamber, an unbalanced load is applied to the rotor and its rotary shaft, thus decreasing the durability and the operational reliability of the moving parts of the pump. In addition, such construction produces noises that make this pump impractical.
A balanced vane type oil pump free from these problems is disclosed in U.S. Pat. No. 2,887,060. In this patent a pair of pump chambers, formed about a rotor at positions symmetrical with respect to the axis of the rotor, are connected with two independent discharge passages which open into the respective pump chambers at positions symmetrical with reference to the rotor axis, so that the single rotor pump acts as two independent pumps. This construction, however, increases the number of fluid flow passages and complicates the connections of these passages to the spool valves of the control unit.
Since the pump cartridges, control unit, and flow passages are incorporated into a single pump body to form an oil pump, the problems described above have a large influence upon the manufacture, assembly, and cost of the pump and tend to increase the size of the pump.
It is desirable for a pump of this type to be of a simple construction, easy to assemble, and to have a small size and weight as well as low cost. These characteristics are advantages when the pump is to be installed in a small space in an engine compartment, for example, to drive a power steering apparatus. Accordingly, development of an energy saving type oil pump that can satisfy all of these requirements sought is needed.